Fuel cells represent one of the most technologically promising solutions for the use of hydrogen as an energy source. They are devices which can, by taking advantage of an electrochemical reaction, convert chemical energy into electric power.
In a single PEM cell there take place simultaneously two half-cell reactions, at the anode and at the cathode respectively. Anode and cathode of a PEM fuel cell are separated by an electrolyte, typically consisting of a membrane of a sulphonated polymer capable of conducting protons, whose opposite sides are coated with a layer of a suitable catalytic mixture (e.g. Pt-based).
The electrolyte is generally saturated with an ionic transportation fluid (e.g. water) so that hydrogen ions can travel thereacross from anode to cathode.
The overall reaction taking place in a fuel cell is:2H2+O2→2H2O   (1)which is accompanied by the development of heat and electric energy and which results from the sum of two half-cell reactions occurring, respectively, at the anode:2H2→4H+4e″  (2)and at cathode:O2+4H+30 4e″→2H2O.   (3)
At the anode, then, hydrogen is supplied which diffuses within the catalytic layer and dissociates into hydrogen ions and electrons, which, the membrane being impermeable to them, travel across an external electric circuit towards the cathode, generating an electric current and the corresponding potential difference. At the cathode, a gaseous mixture containing oxygen is supplied that reacts with the hydrogen ions which have travelled across the electrolyte and the electrons coming from the external electric circuit.
It is necessary that the reacting gases be humidified because it is thanks to the water molecules that the passage of protons across the polymeric membrane occurs: too low a degree of humidification leads to a reduced passage of protons from the anode compartment to the cathode compartment with a consequent worsening of the fuel cell performance, whereas too high a degree of humidification may cause the occlusion of the catalytic sites with a consequent deterioration of the fuel cell performance.
Since a well defined voltage is associated with reaction (I), in order to achieve higher voltages, a plurality of elements are generally connected in series to form a stack.
In addition to the stack, a fuel cell electric generator designed for back-up in the absence of network electricity supply comprises a hydraulic circuit (pump, piping, dissipators, etc.), gaseous currents feed and discharge circuits (hydrogen feed piping, oxygen feed piping, etc.), a control system (control unit, temperature, flow and pressure gauges, actuators, etc.). The ensemble of all the above elements constituting the remaining part of the fuel cell generator, reference is made, here and in the following, as BoP (i.e. “Balance of Plant”).
The ensemble of all the elements forming hydraulic connections between the stack passageways and other major elements of the fuel cell electric generator (e.g. the sources of reagents) generally occupy a significant space and contribute largely to the overall weight of the system. Further, the time needed for their assembling represents a significant part of the time needed to assemble the whole system. In turn, this assembly time directly affects the overall cost of the fuel cell electric generator. FIG. 1 illustrates a view of a known fuel cell electric generator wherein the stack is connected to a plurality of conduits through which reagents are fed to the fuel cells and reaction products are split into two currents, one of which is re-circulated to the stack while the other one is ultimately expelled from the system.
Solutions are known, however (e.g. from U.S. Pat. No. 6,541,148), which partly tackle this drawback by providing the fuel cell generator with a manifold body which communicates flows with the stack and further comprises a separator, located in the manifold body itself, to collect water from at least one of the flows, thus reducing, to a certain extent, the overall bulkiness.
Further, U.S. Pat. No. 6,875,535 teaches providing such a manifold with a plurality of ports and fluid passages adapted to accommodate monitoring devices to monitor fluid condition.
Such an arrangement including a manifold body may be particularly suitable for systems wherein the humidity of the reagent flows fed to the fuel cells is maintained within a desired range of values compatible with the proper operation of the generator by circulating back to the stack a predetermined portion of the exhaust anode and cathode flows. In this case, the manifold body may comprise mixing rooms in which dry fresh reagent flows are mixed with re-circulated exhaust flows carrying part of the water produced by the electrochemical reaction occurring in the fuel cells.
However, in this arrangement, humidity regulation and control is complicated by a number of factors.
Firstly, fresh reagents fed to the anode and cathode compartments undergo expansion prior to reaching their respective mixing rooms in the manifold. As a consequence, their temperature decreases, very often dropping below room temperature.
As a result, when these dry and cold flows are mixed with the humid re-circulated exhaust flows in the manifold body mixing rooms, their low temperature may cause excessive condensation of the water carried by the exhaust flows, thus undesirably reducing the resulting humidity of the flow leaving the manifold body to enter the stack, which may thus be too dry to ensure fuel cell humidification degrees compatible with the correct operation of the stack. External humidification means are therefore needed, which disadvantageously increase the complexity of the BOP and affect the time needed for the installation and routine maintenance thereof. Further, the higher the number of variables, the more complex the control system supervising the operation of the electric generator, which is disadvantageous both in terms of increased costs and reduced reliability.
Secondly, the fuel cell stack is not only part of the hydraulic circuit delivering the gaseous reagents thereto and the exhaust flows to respective recirculation loops and drainers of the anode and cathode compartments, but it is integrated within a second hydraulic circuit wherein a coolant fluid flows to remove the heat generated in the stack by the electrochemical reaction and then yields it to a flow of yet a further cooling fluid, or to cooling means such as radiators and the like, or to a combination thereof.
In this respect, the thermal flows involving the stack and the coolant need to be thoroughly controlled and monitored. Because the coolant fluid is in direct contact with the stack elements, in fact, an uncontrolled increase in its pressure thereof may damage the fuel cells at a structural level. As a consequence, the coolant fluid pressure has generally to be maintained below a predetermined safety value, especially when, upon removal of heat from the stack, its temperature is expected to rise.
To this end, a further expansion vessel for the coolant needs to be provided. Disadvantageously, such a solution renders the system structure more cumbersome and further complicates the already delicate thermal integration balance. This, in turn, has repercussions, as described above, also on the humidification of the flows fed to the stack.
As can be immediately inferred from what described thus far, the known embodiments of back-up electric generation systems are relatively expensive and require accurate and careful routine maintenance to prevent their becoming unreliable. It is worth reminding that, since they are back-up systems, i.e. they are designed for intervening only occasionally, hence all moving parts (e.g. pumps, compressors, etc.) need regular and accurate controls so as to not be out of order just when an emergency occurs.